Although it might seem to be a reasonable option, Dell has so much proprietary hardware in most of the PC's that it's more work than is worthwhile. In the long run, it's better to sell it for what you can get, as is, and start fresh with a "White Box" that contains ATX-standard components.
But if you insist, we'll do this alphabetically:
The front panel connectors are almost always in an "enbloc" single plug that will have to be laboriously separated out into the more normal connectors.
The heat sink unit is an integrated device that works with the case, a special shroud, and a proprietary heat sink. (And, the CPU may be in a nonstandard socket also.)
The motherboard's main power connector may be proprietary, requiring a special plug from the power supply.
Dell sells some truly odd shaped MB's in many of their PC's. These things have the add-on AGP and PCI slots removed from the MB itself, and moved onto a daughter board that sits in the bottom of the case, resulting in all of the slots being parallel with the MB, not perpendicular, so an ATX case won't fit.
The power supply is almost certain to be the bare minimum power to handle the system as it was delivered, so a replacement would be needed to improve the VGA capabilities by much of a margin.
Very often, the psu is potentially proprietary also, but even if not, the power plug has to be changed to use an ATX psu.
There's more, I'm sure, but I need to get a sandwich made. I'm feeling as if I'm starving now.
😕
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